1. Field of the Invention
The present invention generally relates to modulation of light and in particular to extremely fast modulation in which one light beam controls the passage of a second light beam through a coupling medium.
2. Description of Prior Art
Presently, one problem limiting the modulation of light with light at a high speed is the gradual change or degradation of the medium used to couple the modulating light with the modulated light. This undesired medium change can be a rise in temperature, even to the extent of a change in phase such as liquid to gas or solid to liquid, or the gradual loss with time of a coherence between a lower and upper electronic energy level of the medium, or a buildup of a proportion of the medium in any undesirable state.
The modulation of light with light has been proposed previously and several different means for interacting with a medium have been suggested. Such methods are reviewed by R. W. Keyes and J. A. Armstrong in "Thermal Limitations in Optical Logic", Applied Optics, Vol. 8, No. 12, December 1969. This paper estimates a fundamental limitation of 100 W/cm.sup.2 or less for the possible rate of heat dissipation. The rate of heat dissipation is shown by Keyes et al to limit the speed at which light controlled light modulators are capable of continuously modulating since the power essentially required always increases with modulation speed for the known mechanisms, as shown in the above paper.
One mechanism for light controlled light modulation is described by M. A. Duguay in U.S. Pat. No. 3,671,747. Light pulses are controlled by the optical induction of birefringence in solid materials, or the optical Kerr effect. The light pulses are of such high intensity as to degrade the materials suggested if the pulses should be applied continuously. Birefringence can also be induced electronically by the Kerr effect for light modulation.
Another modulation mechanism proposed is the saturation of gain or absorption of an optical medium by the controlling light beam. Embodiments have been proposed by Kosonocky, Doctoral Thesis, Columbia University, 1965; N. G. Basov, Soviet Physics Uspekhi, Vol. 12, No. 2, September-October 1969; and R. W. Gray and L. W. Casperson, "Opto-Optic Modulation Based on Gain Saturation", IEEE Journal of Quantum Electronics, Vol. QE-14, No. 11, November 1978. In these devices, the modulation speed is limited by the thermal degradation of the material involved.
A third mechanism for the modulation of light with light is described by J. A. Armstrong and D. R. Grischkowsky in U.S. Pat. No. 3,864,020. The inventors propose the use of the mechanism of adiabatic following for inducing a birefringence in a material such as an alkali metal vapor by "adiabatically" applying a light beam. The maintenance of the birefringence depends on maintaining a high oscillator strength electronic optical transition in a medium in a combination of lower and upper levels with a particular coherence between them. However, for the same required transitions, the radiative decay of the upper state will limit the lifetime of both the upper state and the coherence between the two, therefore only allowing control of pulses of a time width less than the spontaneous time (of order 10 nanoseconds for the high oscillator strengths proposed) and thus preventing continuous control of any light beam.